a. Field of the Invention
The present invention relates to a compact telephoto lens with an aperture ratio on the order of F/2.0 and a telephoto ratio not exceeding 1.0.
B. Description of the Prior Art
In general telephoto lens systems with large apertures on the order of F/2.0, telephoto ratios can be reduced by enhancing the refractive powers of the front positive lens group. In case where the refractive power is enhanced by this method, however, negative spherical aberration is produced and, accordingly, there occurs an important problem to correct it by the rear lens group to such a degree that the lenses are usable with high aperture ratios.
Among lens systems of such a type, those disclosed by Japanese Pat. Publication Nos. 9468/1975 and 1586/1970 are well known to the public. However, these lens systems have some shortcomings in that they are high in telephoto ratio though being high in their aperture ratio, or that they are low in telephoto ratio but also low in aperture ratio. That is to say, the lens systems can not satisfy the requirements that high aperture ratio is obtainable in combination with a low telephoto ratio in one and the same lens system.
Further, in the above-mentioned type of telephoto lens systems with high aperture ratio, aberrations for short photographing distances are far more aggravated than for infinite photographing distance and this tendency is increased especially when telephoto ratio is selected at lower value. As a measure of favourably correct spherical aberration and curvature of field, it is sufficient to balance the aberrations well for infinite photographing distance. However, such a measure is ineffective to correct asymmetrical coma and therefore increases flare due to coma, thereby resulting in degradation in quality of the image.